79 research outputs found
Reversible Neutral Dissociation of the N-Si Dative Bond in Hexacoordinate Hydrido Complexes of Silicon
No full textHexacoordinate silicon dichelates with hydrido and methyl monodentate ligands were prepared by transsilylation. The temperature-dependent (29)Si NMR spectra of toluene-d(8) or CDCl(3) solutions provide evidence for a reversible, neutral dissociation of the dative N-Si bond, accompanied by a shift of the (29)Si resonance from high field (hexacoordinate) to lower field (pentacoordinate) with increasing temperature. When a phenyl group is attached directly to silicon (replacing methyl), the N-Si dissociation is prevented, presumably due to electron withdrawal by the phenyl. Two of the silicon complexes were characterized by crystallographic analysis. Quantum chemical calculations at the RB3LYP/6-31G(d) level confirm the stability of an N-Si dissociated species (1.03 kcal mol(-1) lower in energy than the hexacoordinate dichelate) and rule out the alternative possibility of an O-Si zwitterionic dissociation, thus supporting the mechanism suggested by the experimental results.Israel Science Foundation [ISF-139/05
Penta- and hexacoordinate silicon mixed dichelates with the SiC2O2N(Cl) ligand environment
No full textNew dichelate complexes of silicon with different chelate rings have been prepared by transsilylation, using ClCH2SiMeCl2 and various N- and O-TMS-hydrazides and amides. Their structures and possible transformations between penta and hexacoordinate complexes have been studied. Many of the complexes are pentacoordinate ionic chloride salts, with charges on an ammonium nitrogen or on silicon. Compounds 5a,b have a zwitterionic aminimide structure, with a possible additional positive charge on silicon. 12 Is the only compound in the series in which penta- hexacoordinate complex exchange is found, involving reversible non-ionic N-Sibond dissociation. 16 is the first reported mixed dichelate siliconium-ion salt with two O→Si dative bonds. Its structure, as well as that of 5a, is established by crystallographic analysis
Competing reactions of hypercoordinate silicon dichelates
No full textNeutral hexacoordinate silicon complexes derived from hydrazide chelating ligands with imino-donor groups, and their pentacoordinate ionic dissociation products, undergo facile intramolecular aldol-type condensation catalyzed by their chloride counterion leading to formation of a third chelate ring. In analogous silacyclobutane dichelates, in the absence of halide counterion, a similar uncatalyzed rearrangement takes place, accompanied by opening of the four-membered ring. In the absence of et-protons necessary for the condensation, the four-membered ring residue adds directly to one of the imino-carbon atoms forming a new C-C bond and closing a different chelate ring. This latter addition to the imino carbon is the preferred reaction pathway, even in the presence of 12 alpha-protons, when cyanide ion replaces the chloride counterion and acts as nucleophile. The cyanide reactivity is rationalized in terms of the HSAB concept. An unusual intramolecular rearrangement involving the migration of a t-butyl group from silicon to carbon, while enabling the unprecedented attachment of a third hydrazide chelating agent, leading to a hexacoordinate trichelate complex, is presented. Copyright (C) 2008 John Wiley & Sons, Ltd.Israel Science Foundation [ISF-139/05]; INTAS [03-51-4164
Donor-stabilized silyl cations. 11. Bis-zwitterionic penta- and hexacoordinate silicon dichelate complexes derived from (ClCH2)(2)SiCl2 through double internal displacement of chloride
No full textThe reaction of (ClCH2)(2)SiCl2 (4) with O-Me3Si-hydrazides (3) yields three new types of products: a 1: 2 reagent ratio leads to a neutral hexacoordinate dichelate with two six-membered rings each containing an N+-N- ylide structure, as well as to the corresponding dissociated pentacoordinate dichelate siliconium chloride salt. These products readily undergo partial hydrolysis to form a dinuclear complex: dicationic disiloxane dichloride. All three product types have been characterized by means of single-crystal structure analysis. Nonempirical ab initio calculations were carried out to determine the atomic charges. Natural bond order (NBO) calculations show that there are no significant charge accumulations on either the ammonium or the imidate nitrogen in the ylide moieties and that the net charge on silicon (which could formally be -2) is between +1.8 and +2.1 e in all species, regardless of overall molecular charge
Competitive molecular rearrangements in hexacoordinate cyano-silicon dichelates
No full textPentacoordinate siliconium chloride or neutral hexacoordinate silicon complexes with imino-nitrogen donor groups react with cyanotrimethylsilane in two competing reactions, leading either to addition of the cyano group to the imino carbon or to hexacoordinate cyano-silicon complexes. The latter may further transform to a rearranged tricyclic pentacoordinate complex. The common driving force for these reactions seems to be the conversion of one of the two initial N -> Si dative bonds, present in the starting complexes, to a shorter formal covalent bond
Ligand Exchange via Coordinative Si-N Bond Cleavage and Pseudorotation in Neutral Pentacoordinate Silicon Complexes
No full textLigand Exchange via Coordinative Si-N Bond Cleavage and Pseudorotation in Neutral Pentacoordinate Silicon Complexes
No full textMolecular Design, Nuclear Molecular X-Ray Radiography and Structure of New Compounds of Hypervalent Silicon - N- and O-Sililic and -Sililmethylic Derivatives of Amides and Hydrazides
No full textAvailable from VNTIC / VNTIC - Scientific & Technical Information Centre of RussiaSIGLERURussian Federatio
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